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Highest 24-Hour PM2.5 Concentration

PM2.5 is very fine particulate matter, 2.5 micrometers and smaller.  This indicator measures the 3-year running average of the 98th percentile 24-hour average concentration of PM2.5. The highest PM2.5 concentrations in Tahoe are associated with wildfires. Particulate matter less than or equal to 2.5 microns in size (PM2.5) is associated with a variety of human health concerns, including asthma, lung diseases, and heart disease. The primary sources of PM2.5 in the Tahoe Basin are residential fuel combustion, campfires, wildfires, prescribed fires, motor vehicles, and dust from unpaved roads. PM2.5 is monitored by TRPA and partners at three sites around the Basin.

24-Hour Pm2.5 Concentration.jpg

Status

PM2.5 monitored in the Tahoe Basin from 1990 to present. The 98th percentile averaged over the three year period is plotted for each year. 

Data provided by the Desert Research Institute. Access detailed datasets on Tahoe Open Data, including:: Air Quality Station Location Data. Daily Monitoring Data. Yearly Average Data.

Evaluation Map

All locations of air quality monitoring stations in the Lake Tahoe Basin. PM2.5 is monitored by TRPA and partners at three sites around the Basin including D.L. Bliss, Lake Tahoe Community College, and Tahoe City..

2023 Evaluation

Status
Considerably Better Than Target
Trend
Moderate Decline
Confidence
Moderate
View Evaluation

Applicable Standard

AQ11: Particulate Matter2.5 24-hour Standard - Maintain Particulate Matter2.5 at or below 35µg/m3 measured over a 24-hour period using gravimetric or beta attenuation methods or any equivalent procedure which can be shown to provide equivalent results at or near the level of air quality standard.

Key Points

  • This indicator reports the three-year average of the annual 98th percentile. The average was above the level of the standard for the first time in 2020 and has continued to exceed it.
  • The Tahoe Region continues to enjoy high air quality, but is increasingly affected by wildfires. See wildfire events effecting the Lake Tahoe air basin at the Climate Resilience Dashboard. 
  • The increasing extent and severity of wildfires is contributing to the increasing trend in peak PM2.5 concentrations. The highest 24-hr concentration of PM2.5 is heavily influenced by wildfires. 
  • Lake Tahoe experiences long-term improvement in the average annual concentrations of particulate matter, but an increase in peak concentrations of particulate matter.
  • The best scenario for air quality on a local, regional, and national scale is the restoration of the natural fire cycle through prescribed fire or managing natural ignitions when conditions are favorable (Schweizer et al. 2019)

About the Threshold

This indicator measures the 3-year running average of the 98th percentile 24-hour average concentration of PM2.5. Particulate matter less than or equal to 2.5 microns in size (PM2.5) is extremely small and can be inhaled deep into the lungs causing or aggravating asthma, lung diseases, and heart disease. Some particles pass into the bloodstream and some are considered carcinogens.

Particulate matter pollution consists of very small liquid and solid particles in the air. The primary sources of PM2.5 in the Lake Tahoe Region are residential fuel combustion, wood smoke from wildfires and prescribed fires, motor vehicles, and paved and unpaved road dust. PM2.5 results from primary emission sources, condensation of semi-volatile organic gases, and from secondary formation from reactions of gases in the atmosphere. Small particles are also transported into the Lake Tahoe Basin, and the ambient concentration of PM2.5 is highly dependent on meteorological conditions such as wind speed and mixing conditions.

Delivering and Measuring Success

EIP Action Priorities

  • Build and Enhance Trail Networks

    Bicycling, walking, and other forms of active transportation are important methods of travel that promote healthy lifestyles, improve air quality.

  • Build and Enhance Transit Systems

    Comprehensive and coordinated bus and shuttle transit system that provides seamless around the lake service.

  • Implement Prescribed Fire

    Prescribed fire and fire used for ecological benefit can reduce smoke exposure from large high intensity fires.

  • Reduce Hazardous Fuels

    Reduce hazardous fuels and proactively manage forests to improve ecosystem resilience and reduce wildfire smoke.

EIP Indicators

Example EIP Projects

Local and Regional Plans

Monitoring Programs

Rationale Details

Considerably Better Than Target. The method for status determination follows State and Federal guidelines for assessing compliance with the 24-hour PM2.5 standard. Sites are assessed as in attainment if the 3-year running average of the 98th percentile 24-hour average concentration of PM2.5 is below 35 µg/m3.
The analysis was conducted both with and without the inclusion of data from periods influenced by exceptional events. Exceptional events are natural or unusual events that can overwhelm existing strategies designed to control man-made pollution, such as wildfires. If such an event occurs, data that would cause an exceedance of a standard was excluded when determining compliance with that standard.
The data was analyzed with and without days influenced by wildfire. Days where air quality was impacted by wildfire are listed on the “Fire Dates” tab of the spreadsheet below. Including all days, the three-year average 98th percentile 24-hour average concentration of PM2.5 exceeded the standard of 35 µg/m3 in 2021, 2022, and 2023. Removing days influenced by wildfires, the three-year average 98th 24-hour average PM2.5 never exceeded 20µg/m3. Thus, the status relative to the standard was significantly better than the target in all years of the evaluation.
Moderate Decline. Trend was determined using the 3-year running average of the 98th percentile 24-hour average concentration of PM2.5 reading at all sites. There is an increase of 1.88% compared to the standard, therefore, the trend is considered moderate decline.

Confidence Details

High. Monitoring was conducted using strict protocols to collect a long-term, high quality dataset.
Low. Confidence of trend is based on the coefficient of determination and t-test significance. R²=0.21, p-value= 0.00618.
Moderate. If one confidence rating is high and the other is low, the overall confidence rating is moderate.

Additional Figures and Resources

No photos available.

Air Quality Exceedances with Exceptional Events
Uploaded On
1/17/2025
File Type
Excel (XLSX)
Description
This document includes sheets for PM2.5, PM10, O3, and exceptional event dates. It shows exceedance values and processed values with and without exceptional events, as well as relevant Threshold Standards . Columns ending in "_exempt" exclude exceptional events, while "_all" includes all dates.